Thermoplastic substrates such as polycarbonates are generally characterized by many advantageous properties which include clarity, high ductility, high heat deflection temperature, as well as dimensional stability. Many of these materials are transparent and are conventionally employed as replacements for glass in commercial applications. However, they often are susceptible to degradation by ultraviolet light. This results in unfavorable characteristics including yellowing and erosion of the substrate surface.
Recently, it is becoming more and more common for thermoplastic substrates such as polycarbonate to be employed outdoors. It is thus important to impart weatherability properties to the substrate. This is often accomplished by treating the substrate surface with a weather resistant coating material, whereby the coating material typically contains ultraviolet light absorbing agents. Weather resistant coating systems can be prepared by incorporating ultraviolet light absorbers, such as benzotriazoles and benzophenones, and hindered amine light stabilizers.
It is often discovered, however, that the ultraviolet light absorbing compounds (herein also referred to as UV absorbers), themselves, decompose upon exposure to ultraviolet light. Prolonged exposure to sunlight, moisture and thermal cycling conditions can cause yellowing, delamination and formation of microcracks in the coating material, decreasing transparency. This leads to a degradation of the favorable properties of the thermoplastic substrate which the UV absorbers are originally employed to protect. Thus, there is an ongoing need to seek new, efficient UV absorbing compounds.
Recently, 4,6-dibenzoylresorcinols (DBR) have been found to be outstanding UV absorbers in coatings due to their remarkable photostability, as described in co-pending and commonly assigned application, Ser. No. 08/669,899, titled "COATINGS USEFUL FOR ABSORBING ULTRAVIOLET LIGHT" now U.S. Pat. No. 5,869,185. Also, DBRs as bulk additives has been described by Gordon and Hudson in U.S. Pat. Nos. 2,794,052 and 2,933,553, assigned to Dow Chemical Company. However, a drawback in current compositions of DBRs is their relatively low molecular weight, i.e., molecular weight less than or equal to about 400 daltons, which leads to volatility during processing of the thermoplastic substrates or cure of the coatings. This can result in fouling of the processing equipment. Efforts to increase the molecular weight of DBR by adding substituents can result in diluting the effectiveness of the UV absorber. There is a need for novel UV absorbing compounds that retain or enhance photostability and effectiveness of the DBR chromophore while increasing the molecular weight of the compound. There is also a need to have UV absorbers with low volatility.